If your Windows drive is mysteriously running out of space, there are plenty of obvious suspects. Your Downloads folder. The Recycle Bin. Some enormous game you forgot you installed. A number of things. But there is another folder hiding inside Windows that can quietly grow to several gigabytes - and sometimes considerably more: 
C:\Windows\Installer
Before you open it and start deleting things, however, there is one very important rule:

Do not just delete everything in here

The files in this folder aren't ordinary temporary files that Windows forgot to clean up. Many of them are part of the Windows Installer cache and may be required later when software needs to be repaired, updated, modified, or uninstalled. So yes, you can potentially reclaim a lot of disk space here. But no, the solution is not to select everything and hit Delete. Let's take a look at what this mysterious folder actually does and what you can do when it has grown large enough to start competing with your Steam library.

What is C:\Windows\Installer?

C:\Windows\Installer is a hidden system folder used by Windows Installer. When you install software using Microsoft's MSI-based installation technology, Windows can place installation packages and patches in this directory. You will commonly find files with extensions such as:
  • .msi - Windows Installer packages
  • .msp - Windows Installer patches
Windows uses this cached information when software needs to be updated, repaired, modified, or removed. That means the files aren't necessarily useless just because you haven't opened the folder in years. In fact, that's the whole point of the cache - Windows keeps copies around so it doesn't have to go hunting for the original installation media later.

Why does it get so large?

Software gets installed, updated and removed over time, while the Installer cache can accumulate files along the way. If you've had the same Windows installation for several years and regularly install desktop applications, there may be a surprisingly large collection of installer packages sitting there. Programs that receive frequent updates can contribute particularly large amounts of data. And unlike your Downloads folder, where you can usually look at a file and decide whether you still need it, the files in C:\Windows\Installer aren't exactly named definitely-not-needed-anymore.msi. They're generally identified using names that aren't particularly helpful to humans. 
123456.msi
789012.msp
1a2b3c.msi
Good luck figuring out which one belongs to the application you haven't opened since 2021.

How much space can it use?

There's no fixed size limit you should expect. On one computer the folder might be relatively small. On another, it can consume several gigabytes. On a Windows installation that's been around for a long time and has accumulated a lot of MSI-based software, it can become considerably larger. That's why it's worth checking the size before doing anything. You can open the folder by typing this into the File Explorer address bar: 
C:\Windows\Installer
Windows normally keeps the folder hidden, but entering the path directly will take you there. As said - Don't use that as an invitation to start deleting files, though.

Why deleting the files manually is a bad idea

This is where a little bit of Windows knowledge can save you from a much larger headache. Some of the cached installation files may still be required by applications installed on your PC. If you simply delete them because they look old (by date, not dust), you can end up with software that can no longer be properly updated, repaired or uninstalled. You might not notice anything immediately, which makes this particularly nasty. Everything can appear to work perfectly until six months later when an application needs one of those cached files and Windows discovers that you've enthusiastically cleaned it out - But you'll have forgotten all about it. At that point, your innocent attempt to free 8 GB can turn into: 
Windows Installer
The feature you are trying to use is on a network resource that is unavailable.

So how do I safely clean it?

This is where we need to distinguish between cleaning Windows and manually deleting the Windows Installer cache. Windows does provide several built-in mechanisms for removing files that are genuinely considered unnecessary, but there isn't a simple "delete all unused files from C:\Windows\Installer" button that you should blindly replace with manual deletion. Instead, start with the boring solutions first.

1. Uninstall software you don't use

This is probably the safest place to start. Open: 
Settings -> Apps -> Installed apps
Go through the list and remove applications you no longer use. If you've got five different PDF readers, three old development tools, two launchers for games you haven't played since 2022 and some random utility you installed because a YouTube video told you to - this is your moment. Removing software can also remove installation data associated with that software, depending on how the application was installed and maintained.

2. Let Windows clean what Windows knows is safe

Windows includes its own storage-management tools that can remove various categories of unnecessary files. Go to: 
Settings -> System -> Storage
From there, review Temporary files and the other storage categories Windows offers. You can also enable Storage Sense so Windows can automatically clean up certain types of unnecessary files. Just don't confuse Storage Sense with a magical "clean every file Windows has ever cached" button. It isn't one. Read more about Storage Sense in our article here.

But what if C:\Windows\Installer is REALLY huge?

If you've checked the folder and discovered that it is consuming something like 20, 30 or even 50+ GB, some people even claiming theirs is 200+ GB. I will say, on my Windows install that's a couple of years by now, mine is using shy of 3 GB. You may be tempted to find a cleanup utility specifically designed to identify unused MSI and MSP files. That can be useful, but this is where you need to be careful about what software you trust. A tool that understands Windows Installer metadata can potentially determine whether cached packages are still referenced by installed applications. A tool that simply looks at file dates and decides that anything old is disposable is a very different story. Old and dusty does not necessarily mean unused, since "this file hasn't been touched in three years" is not a valid reason to delete a Windows Installer package.

Don't use Explorer as a cleanup algorithm

Especially inside 
C:\Windows
it's dangerous to think "I don't know what this folder is, but it's taking up too much space, I'll delete it". Windows contains a lot of files whose purpose isn't obvious from their names. Some are caches, some are backups, some are databases, some are installation resources and some are things that Windows absolutely expects to still be there later.

Need more space? Start somewhere else

Before touching system folders, check the easy wins.
  • Empty the Recycle Bin.
  • Clean up your Downloads folder.
  • Remove old Windows Update and temporary files using Windows' storage tools.
  • Uninstall applications you don't use.
  • Remove old installers and ISO files you no longer need.
  • Check whether your game library is eating half the SSD.
That last one is especially important for some of us. If you're running out of space because you have six versions of Call of Duty, three MMORPGs and 400 GB of Steam shader caches, deleting random MSI files probably isn't the first place I'd look.

The bottom line

C:\Windows\Installer is a legitimate Windows system folder that can become surprisingly large over time. It contains cached Windows Installer packages and patches that applications may need later for maintenance operations such as updates, repairs and uninstallation. Don't manually delete everything inside it. Instead, use Windows' built-in storage cleanup tools, uninstall software you no longer need, and investigate unusually large Installer caches carefully rather than treating the folder like a normal pile of temporary files. If the folder is consuming an absurd amount of space and the normal cleanup methods aren't enough, that's the point where a proper Windows Installer-aware cleanup tool makes sense.